Micro-mechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits. Micro electromechanical systems (MEMS) have both electrical circuitry and mechanical systems integrated in a single device. Micro optical-electromechanical systems (MOEMS) are a subset of MEMS that have optical components integrated with the electromechanical systems.
Micromechanical devices include micromirror devices, accelerometers, pressure and flow sensors, gears and motors. While some micro-mechanical devices, such as pressure sensors, flow sensors, and micromirrors have found commercial success, other types have not yet been commercially viable.
Micromirror devices are primarily used in optical display systems. In display systems, the micromirror is a spatial light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, micromirrors typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics were used to illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the micromirror surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
The deflectable portion of the micromirror device is prone to sticking to the landing portion of the underlying metal layer. There are several mechanisms that create this stiction force, including cold welding and van der Waals force. To lessen the likelihood of sticking, a lubrication layer, sometimes referred to a passivation layer, is used. Micromirrors typically use a perfluorodecanoic acid (PFDA) monolayer to prevent the deflectable members from permanently sticking to the landing zones.
While much has been done to improve the mechanical performance and reliability of micromirror devices, there still is a need for methods and systems of improving the reliability of the device and the contrast of the images produced by the devices.